1. Field of the Invention
The present invention relates to a method of learn-controlling an air-fuel ratio for an internal combustion engine, particularly a spark-ignition engine, and more particularly, to a method of learn-controlling the air-fuel ratio by the use of a closed loop.
2. Description of the Prior Art
Hitherto, a three-way catalyst is employed for simultaneously purifying carbon monoxide, hydrocarbon and nitrogen oxide in an exhaust gas. In order to better the purifying efficiency of the three-way catalyst, such a closed loop control has been proposed and used that an air-fuel ratio is estimated by detecting the residual oxygen concentration in an exhaust gas by means of an O.sub.2 sensor and is controlled so as to be in proximity of a stoichiometric air-fuel ratio. This closed loop control is effected in such a way that a fuel injection time duration TAU is obtained by multiplying a basic fuel injection time duration TP, which is determined by an engine load (an intake-pipe pressure PM or an intake-air quantity Q/Ne per revolution of the engine shaft) and an engine speed, by an air-fuel ratio feedback correction coefficient FAF, shown in FIG. 1, for allowing the fuel injection time duration to perform a proportional-plus-integral action in accordance with an air-fuel ratio signal which is delivered from the O.sub.2 sensor and signal-shaped, and a fuel injection valve is opened for a period of time corresponding to the fuel injection in time duration TAU, thereby to control the air-fuel ratio so as to converge in proximity of the stoichiometric one. However, any change in environment or change with time may cause the variation of the valve timing due to the fluctuation in tappet clearance, as well as the change in characteristics of a pressure sensor, an air-flow meter and the fuel injection valve, so that it may become impossible to control the fuel injection quantity to a required fuel injection quantity of the engine, and the air-fuel ratio cannot be controlled so as to be in proximity of the stoichiometric one. For this reason, it is a conventional practice to correct the basic fuel injection time duration TP by employing correction values TAUG, KG which are corrected by learning under predetermined conditions as represented by the following equation: EQU TAU=(TP+TAUG).multidot.KG.multidot.FAF.multidot.(1+F)+.tau..sub.v ( 1)
where, TAUG represents a correction value which is corrected by learning during the period when a throttle valve is at fully closed position, KG a correction value which is corrected by learning during the period when the throttle valve is open, .tau..sub.v a non-effective injection time duration applied to voltage correction. And F a correction coefficient which is employed during a transient state, such as a quick acceleration of the engine. Further, the correction value KG is determined in accordance with the engine load: for example, correction values KG.sub.1, KG.sub.2, and KG.sub.3 are employed when the intake-pipe pressure is 200 to 300 mmHg, 300 to 400 mmHg and 400 to 500 mmHg, respectively.
These correction values TAUG, KG are corrected by learning every time the correction coefficient FAF skips during the air-fuel ratio feedback control and when the engine cooling water temperature exceeds a predetermined value (70.degree. C., for example) by the following method. First of all, every time the air-fuel ratio feedback correction coefficient FAF skips, an arithmetic means value FAFAV of peak values of the correction coefficient FAF is obtained as follows: ##EQU1## When FAFAV takes a value out of a predetermined range (.+-.2%, for example), values shown the Table below are added to the pertinent correction values, respectively.
TABLE ______________________________________ TAUG KG ______________________________________ 1.02 &lt; FAFAV +.DELTA.A (.mu.s) +.DELTA.K 0.98 .ltoreq. FAFAV .ltoreq. 1.02 0 0 FAFAV &lt; 0.98 -.DELTA.A (.mu.s) -.DELTA.K ______________________________________
Then, the correction values TAUG, KG thus corrected by learning are applied to the above-mentioned equation (1) in accordance with the opening/closing state of the throttle valve and the magnitude of the intake-pipe pressure (or the intake-air quantity per revolution of the engine shaft), thereby to obtain the fuel injection time duration TAU. As a result, when the mean value FAFAV exceeds a predetermined value (1.02), the correction values are increased to control the air-fuel ratio to the richer side, and when the mean value FAFAV is less than a predetermined value (0.98), the correction values are decreased to control the air-fuel ratio to the leaner side, thereby to control the mean value FAFAV so as to converge in proximity of 1, that is, the stoichiometric air-fuel ratio.
In this conventional method, however, since the correction values are stored in a backup RAM or the like in the form of digital values, the number of bits with respect to the correction values cannot be increased on the grounds of the number of words in the backup RAM. Therefore, even when LSB (least significant bit) is corrected by learning, a correction value has a large change. In addition, since correction by learning is made every skip, a correction value may be corrected into an abnormal value in a short period of time. In consequence, the air-fuel ratio may be instable, disadvantageously. For example, if a correction value is stored in an eight-bit memory area in the backup RAM, 1 LSB, that is, resolution takes a considerably large value, i.e., 1/256 (0.4%). As a result, the air-fuel ratio is greatly varied by the value of 1 LSB, disadvantageously. To obviate this disadvantage, such a means may be employed that an offset is provided too allow the correction value 1.0 to be 512 and the range of change of the correction value to be +128. However, even in this case, 1 LSB is 1/512 (0.2%), so that the quantity of change of the air-fuel ratio becomes large similarly to the above, unfavorably.
For the above reason, correction by learning is made when the mean value FAFAV is beyond a predetermined range owing to an increase in fuel injection quantity for acceleration of the engine, for example, and when the engine operation is returned to the stationary state, the fuel injection time duration is determined by the value corrected by learning as above, so that it becomes impossible to control the air-fuel ratio so as to converge in proximity of a stoichiometric one.